[JS] Is my window open already?

I open an information input window via JS with

Code:

---

myH = window.open(url,"Windowname",'width=700,height=900');

---

That works dandy. and the user can fill it out, and I have it passing information back
to the calling webpage and it is a happy world.

The sad part is that I can't seem to figure out how the 'parent' webpage is supposed to know if there is
an instance of this 'child' window open already.

Cause I don't want to a.) open another instance and basically erase anything the user has already input, or b.) allow the user to continue on the calling page until the input page has been closed.

How do I ensure this stuff?